The present invention relates to so-called deep bed media filters, particularly of the type that include a plurality of filter units arranged or connected as a filter system or installation.
U.S. Pat. No. 5,277,829, which issued on Jan. 14, 1994 to the assignee of the present invention, discloses a deep bed filter unit in which the infilt flows upwardly through a sand bed such that the filtrate accumulates above the sand bed while the dirty sand is continuously regenerated. A plurality of such units can be connected by a common inlet manifold and a common outlet manifold, to form a system or installation, e.g., at a wastewater treatment plant. Another type of regenerative deep bed filter unit in which the infilt flows upwardly, is disclosed in U.S. Pat. Nos. 4,123,456 and 4,126,546. These patents also shows a plurality of modular units clustered to form a system or installation.
In northern areas of the United States (and other regions of the world that experience frozen ground during winter or other ground instabilities such as flooding or earthquakes), the filter systems are preferably supported above ground, on stilts or the like. In areas of more stable temperature and other ground conditions, it may be desirable to wholly or partially embed the filter systems in the ground. A major advantage of the latter arrangement, is the lower cost associated with pouring large concrete basins into a ground excavation, rather than forming and shipping vessels made of steel to be supported by stilts. One drawback of the poured concrete basin, however, is the difficulty some contractors face in properly contouring the basin, especially where the basin is constituted by a plurality of modular vessel units each of which requires its own cylindrical funnel-shaped bottom, as disclosed in said U.S. Pat. No. 4,126,546. These units require tight tolerances for the components to fit properly and to seal against the basin walls.